900 years
In theory radiation will stay in Chernobil area for the next 48.000 years, but in reality first people must start to populate those area already in some 900 years. This is when the most dangerous elements will dissapear. their half-value period is from 300-900 years. I suppose there will be someway discovered to neutralise or clean up the radiation in the next 100 years. Well, if our government will finance our science as they do it now, then we won't be able to rid of this and will have to wait this 900 years untill radiation will evaporate by itself.
Actually, some people coming back to their homes and settle down, those mostly old people who do not care if they die today or tomorrow. important is to die at home.